Get Counts using Linq to SQL - entity-framework

Question

I have the following SQL which works fine
SELECT
f.ForumId,
f.Name,
COUNT(ft.TopicId) AS TotalTopics,
COUNT(fm.MessageId) AS TotalMessages
FROM
tblForumMessages fm INNER JOIN
tblForumTopics ft ON fm.TopicId = ft.TopicId RIGHT OUTER JOIN
tblForums f ON ft.ForumId = f.ForumId
GROUP BY f.ForumId, f.Name
That I'm trying to convert to Linq.
Here's what I have
var forums = (from f in Forums
join ft in ForumTopics on f.ForumId equals ft.ForumId into topics
from y in topics.DefaultIfEmpty()
join fm in ForumMessages on y.TopicId equals fm.TopicId into messages
from x in messages.DefaultIfEmpty()
select new { f.ForumId, f.Name, y.TopicId, x.MessageId } into x
group x by new { x.ForumId, x.Name } into g
select new
{
ForumId = g.Key.ForumId,
ForumName = g.Key.Name,
TopicCount = g.Count(i => i.TopicId),
MessageCount = g.Count(i => i.MessageId)
}
).ToList();
I'm getting an error on TopicCount = g.Count(i => i.TopicId) saying Cannot convert expression type 'System.Guid' to return type 'bool'.
What am I missing to make this work?
Thanks
* EDIT *
Thanks to Rob I got it to work but the counts were always returning 1 for Topic Count and Message Count even though there were no records. It should have been returning 0 for both.
I've modified the query by changing
select new { f.ForumId, f.Name, y.TopicId, x.MessageId } into x
to
select new
{
f.ForumId, f.Name,
TopicId = y != null ? y.TopicId : (Guid?)null,
MessageId = z != null ? z.MessageId : (Guid?)null
} into x
And for the actual counts, I changed the query to
select new
{
g.Key.ForumId,
g.Key.Name,
TopicCount = g.Count(t => t.TopicId != null),
MessageCount = g.Count(t => t.MessageId != null)
}

Answer 1

The offending article is TopicCount = g.Count(i => i.TopicId). The Count method takes a Func<T, bool> (it gives the number of items in the collection that satisfy the predicate).
It looks like you want the number of distinct TopicIds in your group. Try replacing TopicCount = g.Count(i => i.TopicId) with TopicCount = g.GroupBy(i => i.TopicId).Count().

Answer 2

You can also try TopicCount = g.Select(i => i.TopicId).Distinct().Count()